With rapid development of optical communication technologies, WDM (Wavelength Division Multiplexing) network is increasingly widely applied. The WDM is a technology for sending multiple beams of laser light with different wavelengths on a single optical fiber by using multiple lasers. To ensure quality of service data in an optical communication process, an OTDR (Optical Time Domain Reflectometer) needs to be used to detect a loss of a physical link in the WDM network. The OTDR is a precise photoelectric integrated instrument that is produced according to back scattering generated during transmission of an optical signal in an optical fiber, is widely applied to maintenance and construction of optical cable lines, and can be used to perform measurement on an optical fiber length, optical fiber transmission attenuation, and splice attenuation, measurement for fault locating, and the like.
A basic principle of measuring optical fiber transmission attenuation by using a conventional OTDR is: a transmit end of the OTDR sends a single pulse; after electro-optical modulation is performed on the single pulse, the single pulse enters an optical fiber, and Rayleigh scattering and Fresnel reflection are performed on the single pulse; a receive end of the OTDR receives an optical signal emitted from the optical fiber, so that a loss of an optical fiber link is obtained. However, because a power of the sent signal pulse is relatively high, to avoid impact on normal detection of a service signal, the signal pulse cannot be transmitted together with the service signal. Consequently, the loss of the optical fiber link cannot be detected in real time. A basic principle of measuring optical fiber transmission attenuation by using a multipulse OTDR is: a transmit end of the OTDR sends a PN (Pseudo-Noise) code sequence, and combines the PN code sequence and a service signal together for transmission by means of modulation, so that real-time detection on an optical fiber link is implemented. Further, to reduce impact of the service signal on PN code sequence correlation, the OTDR further can modulate the PN code sequence and the service signal to different bands, and distinguish different frequency components by using a filter at a receive end, so as to reduce mutual impact between service data and the PN code sequence.
However, the service signal and the PN code sequence are both baseband signals and can affect each other in a low frequency part. Consequently, the PN code sequence correlation is damaged, performance of the OTDR is seriously affected, and a relatively large detection error is caused. Therefore, the service signal needs to be modulated to a high frequency and be distinguished from the PN code sequence in a frequency domain, in this case, a transmission rate of the service signal is increased, and related hardware needs to satisfy an increased rate of the service signal, causing unnecessary hardware costs and a bandwidth waste.